This invention relates to a solid state image pick-up device.
In a conventional solid state image pick-up device, the charge storage section comprises a charge transfer device such as a CCD, BBD, or the like, and a read-out section to read out the stored charge from this charge storage section comprises an MOS field effect transistor (MOSFET). The output voltage V.sub.out of the output transistor in this read-out section, namely the MOSFET, can be approximately expressed by the following equation when this MOSFET operates in the saturation region. That is, ##EQU1## wherein, I.sub.d : Drain current of the output transistor
V.sub.OG : Gate voltage of the output transistor PA1 V.sub.T : Threshold voltage of the output transistor PA1 .beta.: Current amplification factor of the output transistor PA1 C.sub.F : Capacity of the output gate electrode to the grounded pole ##EQU3## PA1 C.sub.OX : Gate capacity of the output transistor PA1 A: Area of the gate electrode PA1 W: Channel width of the output transistor PA1 C.sub.OX : Unit capacity of the gate electrode PA1 .mu.: Mobility
The sensitivity to the signal charge in response to the output voltage can be expressed by the following equation. ##EQU2## wherein, Q.sub.S : Signal charge
It will be appreciated from this equation that the capacity of the output gate electronic mostly affects the sensitivity. However, the capacity of the output gate electrode is, in fact, a sum of the capacity of the output diffusion layer and the capacity of the gate electrode of the output transistor. It is the capacity of the gate electrode that affects the sensitivity. Therefore, we have EQU C.sub.F .perspectiveto.A.multidot.C.sub.OX
wherein,
It will be understood from equation (1) that it is preferable for R.sub.L and .beta.(V.sub.OG -V.sub.T) of the output transistor, as other essential factors, to have large values. However, it is impossible to obtain a large value for R.sub.L due to the limitation of the read-out speed, and therefore, the value of (V.sub.OG -V.sub.T) cannot be greatly increased. Consequently, it is in fact only a way to increase the value of .beta. and to reduce the value of C.sub.F. In this case, .beta. is expressed by the following equation: ##EQU4## wherein, L: Channel length of the output transistor
From the above equation, it is most effective to reduce the value of L in order to increase the value of .beta.. The value of A will also be reduced by doing this, resulting in the reduction of a value of C.sub.F. A value of C.sub.OX may be reduced to decrease a value of C.sub.F. To reduce a value of C.sub.OX, it may be considered, for example, to increase a thickness of a film oxide. However, an increase in thickness of the film oxide causes a value of .beta. to be reduced, so that this method is undesirable. Therefore, the value of L must be decreased. Although the value of L can be presently reduced to 2 to 3 .mu.m, this only results in an improvement of sensitivity of about 1.5 to 2 times that which is obtained when L is 4 .mu.m.
On the other hand, a high density solid state image pick-up device has been developed recently in which, as the area of a cell becomes smaller, the output signal charge to be detected also decreases. Therefore, while a read-out circuit section with high sensitivity is desired, for the reasons described above, it is impossible to greatly improve the sensitivity in the read-out circuit section using an MOS type transistor.